1. Field of the Invention
The present invention relates to a method of compensating frequency offset of a received phase shift keyed signal which may occur in any of digital mobile radio communication systems such as a digital car telephone system, a digital portable telephone system and a digital cordless telephone system.
2. Description of the Related Art
In a generic phase shift keying system, termed "M-ary Phase Shift Keying" (MPSK), M indicates the number of information phases. Thus, in the case of a Binary Phase Shift Keying System (BPSK) M=2, and in the case of a .pi./4 Quadrature Phase Shift Keying System (QPSK) and a .pi./4 Differential Quadrature Phase Shift System (DQPSK), M=4. M is defined as M=2.sup.N, where N is an integer larger than zero (N&gt;0).
Recently, various new digital radio techniques have been studied, developed and put to practical use, for the promotion of digitizing land mobile communication systems such as a car telephone system and a cordless telephone system. For example, a .pi./4 shift QPSK technique, which is one of several phase shift keying methods having high efficiency of frequency usage, will be adopted for use in a digital car telephone system, a cordless telephone system and a portable telephone system of the next generation.
In the case of a land mobile communication system, since its transmission path will be a Rayleigh fading communication path in which the amplitude and phase of the signals change substantially with respect to time, it is quite difficult to carry out a coherent detection, which includes determination of a reference carrier and a phase-shifted and information signal recovered from the received signal.
For the Rayleigh fading communication path, therefore a differential detection (delay detection) method which does not need the recovered reference carrier but detects the phase difference between two symbols may be advantageously used. However, even in this differential detection method, the bit error rate performance will be deteriorated by phase shift due to frequency offset. Such frequency offset might be caused by instability of an oscillator installed in a transmitter/receiver in mobile stations or in base stations. Many studies and developments with respect to compensation of the frequency offset have been carried out. However, according to the conventional art, the construction of the compensation circuit would become complicated and also the compensable amount of frequency offset would be limited to a frequency offset which corresponds to a phase shift of 2.pi. due to phase ambiguity.
In order to suppress the generation of frequency offset itself in a small value, it is necessary to make frequency oscillators in the base stations and in the mobile stations with high accuracy, causing the manufacturing cost of the oscillators to increase.